1. Technical Field
This invention relates to frequency modulation systems for transmitting voice and data signals and, more particularly, to the detection of noise in such systems.
2. Description of the Prior Art
It is often desirable to have available an indication of the noise content of a signal received through a particular transmission system. In certain forms of transmission systems, the transmission medium changes so rapidly and over such extremes that a constant running indication of noise level is necessary for the proper operation of the system. Such systems include light wave frequency transmission systems, particularly when the transmitter and receiver are movable with respect to one another. When such systems are used for the transmission of voice signals, it has been found desirable to provide a squelch circuit, responsive to an undesirable noise level, for greatly attenuating or squelching the voice signal.
In a hands-free telephone station system in which it is contemplated that the transmitter be portable and carried about with the user as the user moves about the room, the transmission path varies considerably during normal usage. Moreover, if transmission frequencies in the light wave frequency range are utilized to improve the privacy of such a system, the path length of the transmission link does indeed vary considerably. In such systems, instantaneous and continuous noise detection and suppression are essential.
Techniques for deriving a signal indicating the noise level of a received signal are generally discussed in F. M. Gardner's textbook, "Phaselock Techniques." In practice, Celli et al., U.S. Pat. No. 4,228,320, teaches that a phaselock loop circuit comprising an exclusive OR gate provides such an indicator signal in the receiver of their hands-free telephone station system.
A problem, however, has been uncovered in the application of the teachings of Celli et al. The two divide-by-two circuits, employed to provide an in-phase signal of the same frequency for comparison with the received signal, induce an undesirable delay. In conventional operation, the output generated by a voltage-controlled oscillator of the phaselock loop is 90 degrees out-of-phase and ahead of the received signal. The two divide-by-two circuits, while acting to provide the desired in-phase relationship with the received signal, in fact induce delay in the locally generated signal which exhibits itself as a source of undesirable noise. Accordingly, the Celli phaselock loop is unable to detect sufficiently low levels of received signal noise.
Also, in the practice of the Celli hands-free telephone station system, a need was recognized to provide alternate voice or data capabilities. The portable transmitter disclosed in Celli et al. has no capability to remotely request dial tone or dial another telephone. Assuming the receiver may comprise a CRT computer terminal, the Celli transmitter has no capability to manipulate characters on the CRT display screen.